This invention relates to an on/off semi-active suspension control for a wheeled vehicle having a body suspended on an unsprung, road contacting wheel. An active suspension includes an actuator capable of modifying the force between the vehicle body and wheel in response to a control in real time so as to produce a desired suspension behavior. A semi-active suspension is similar but uses an actuator which provides only damping or dissipative forces so that the suspension generates its own force. In addition, an on/off damper has only two operating modes with fixed force/velocity curves: a first mode having a high damping force and a second mode having a low damping force. Such a damper uses a valve capable of switching the damper between its first and second operating modes at frequencies at least twice the resonant wheel vibration frequency to achieve real time control of wheel as well as body motions.
An on/off, semi-active suspension control has been described, at least theoretically, in the prior art in the paper entitled "The Experimental Performance of an `on-off` active damper" by E. J. Krasnicki in the Proceedings of the 51st Shock and Vibration Symposium in San Diego, Calif., in October 1980. This suspension is a modification of the semi-active suspension control previously described by Dean Karnopp et.al. in "Vibration Control Using Semi-Active Force Generators", ASME Paper No. 73-DET-122, June, 1974 and related to the disclosure of U.S. Pat. No. 3,807,678, issued Apr. 30, 1974 to Karnopp et.al. The suspension as described by Krasnicki includes a control which repeatedly calculates a desired actuator force and determines if the power generated by this force would be active or dissipative. If the power would be active, which a damper cannot provide, the control actuates the valve to the second mode of operation to provide minimum (zero) damping. If the power would be dissipative, the control actuates the valve to the first mode of operation to provide high damping.
An improved on/off semi-active suspension control is described in the copending patent application G-5,053, filed Nov. 2, 1989 by Kamal N. Majeed and assigned to the assignee of this patent application. The disclosure of that patent application describes such a control including an on/off damper which is switchable between its high and low damping modes at a frequency at least twice the resonant wheel vibration frequency and preferably higher, perhaps up to 50 Hz. However, response of the damper at frequencies higher than 50 Hz is not necessary for suspension control and would only add to the cost of the system. The control described is embodied in a digital processing system which completes a cycle and provides a new output signal for each on/off damper once every 1 millisecond. Since the damper signal is a single digital bit for selection of high or low damping mode, this corresponds to a maximum output frequency of 500 Hz. It is not desirable to apply a standard low pass filter, which would introduce phase delay, since a fast transient response to the initiation of a large low frequency input is desired for good system response to major road perturbations or vehicle maneuvers. However, on a comparatively smooth road, there is still a significant low amplitude input at high frequencies which can produce a high frequency damper signal (100-500 Hz). This high frequency damper signal, if provided to the damper unprocessed, would result in no useful suspension performance improvement but might cause unnecessary damper valve actuation which could shorten its useful life. Therefore, it is desirable in some instances to filter this signal in a way that limits its frequency while still providing fast transient response.